As natural gas is produced from gas wells, the pressure in the formation will decrease, resulting in a reduction in gas flow rate and associated gas velocity. Before the natural drive pressure is reduced, the flow rate and velocity of produced gas may be sufficient to remove the liquids from the well with the gas. However, at some point the flow rate of gas will be insufficient to carry liquids out of the well. As a result, the liquid loading in the well will increase, and liquid will collect in the bottom of the well, further reducing its output.
When production by natural reservoir pressure becomes uneconomical, artificial lift techniques can be utilized to increase well production. A number of artificial lift systems are known in the industry, including sucker rod pumps, gas lift techniques and plunger lift techniques.
A plunger lift is an artificial lift method used to deliquify natural gas wells and high gas-to-liquid ratio oil wells. A plunger is used to remove contaminants from productive natural gas wells, such as water (as a liquid or mist), oil, condensate and wax. FIG. 1 shows a schematic of a typical plunger system. The plunger cycles between the top and bottom of the well to lift fluids to the surface, as illustrated in FIG. 2. A more detailed graphic of a plunger lift system is also in FIG. 3.
The basic function of the plunge lift controller is to open and close the well shutoff valve at the optimum times, to bring up the plunger and the contaminants, and thus maximize natural gas production. A well without a deliquification technique will stop flowing or slow down and become a non-productive well, long before a properly deliquified well will.
Conventional plunger lift systems, which are also known as free piston systems, utilize a plunger (piston). The well is shut in and the plunger falls to the bottom of the tubing and onto a bumper spring, seating nipple or stop near the bottom of the tubing (FIG. 2, “Off Time”). After pressure in the well has built, the wellhead is opened to flow and the high pressure gas located within the well pushes the piston upward to the surface ((FIG. 2, “Lift”), thereby pushing the liquid on top of the plunger to the surface and allowing the well to produce for as long as possible (FIG. 2, “After flow”). This sequence can be repeated by closing the wellhead off and allowing the plunger to fall again to the bottom of the well while pressure in the well is allowed to rebuild.
A number of patents describe plunger lift systems, including U.S. Pat. No. 5,957,200, U.S. Pat. No. 6,209,637, U.S. Pat. No. 6,467,541 and U.S. Pat. No. 7,270,187. However, each of these relates to minor component improvements or materials improvements, and the basic principles remain unchanged since the earliest patents, such as U.S. Pat. No. 1,769,637. Thus, all rely on pressure from the reservoir to provide the energy needed for the lift operation.
What is needed in the art are improved plunger lift systems with improved efficiency, reduced wear characteristics, and reduced reliance on pressure lifting mechanisms.